The claimed invention involves the use of microorganisms, for example, bacteria in activated sludge, to metabolize organic contaminants in waste water while simultaneously removing nutrients such as nitrogen and phosphates, in conjunction with a membrane to effect the removal of suspended solids from waste water.
Organic compounds in waste water typically have a high biochemical oxygen demand (BOD). Under aerobic conditions, bacteria in activated sludge metabolize this BOD in three ways: (1) substrate oxidation in which organic compounds are converted to carbon dioxide and water; (2) synthesis in which organic compounds and nutrients are converted to cell protoplasm; and (3) endogenous respiration in which protoplasm is converted to carbon dioxide, nutrients and water.
Nitrogen is typically present in waste water in the forms of organic nitrogen and ammonia (NH4+). The process of removing this contaminant requires two distinct steps in which organic nitrogen is hydrolyzed and ammonia is converted to free nitrogen gas (N2) which can readily be stripped from solution to the atmosphere. First, in the nitrification process, ammonia is converted to nitrite (NO2−) by autotrophic oxidation involving Nitrosomonas spp. and related organisms, followed by further oxidation to nitrate (NO2−) involving Nitrobacter spp. Second, in the denitrification process, a relatively broad range of heterotrophic facultative organisms convert nitrate to free nitrogen (N2) in a series of steps.
It is apparent that the nitrogen removal process, as described above, requires first an aerobic step in which the oxidation of ammonia to nitrate takes place (nitrification), followed by an anoxic step in which facultative organisms convert nitrate and nitrite to free nitrogen which can be released (denitrification).
The removal of phosphates takes place in two steps and is mediated by a group of phosphorous rich microorganisms (Bio-P), principally Acinetobacter spp. and some Aeromonas. These organisms, when present in sludge exposed to anaerobic conditions, use stored energy in the form of poly-phosphate to absorb food materials, for example, volatile fatty acids such as acetic, propionic, or butyric acids formed in the activated sludge as a result of exposure to anaerobic conditions) and store it as poly-B-hydroxybutyrate (PHB). In the process, the organisms release phosphates as the polyphosphates are broken down to release energy. The conditions must be anaerobic rather than anoxic to allow for the depletion of nitrates which would inhibit phosphate release and the absorption of volatile fatty acids by the microorganisms.
In the second step of phosphate removal, under aerobic conditions, the aerobic bacteria contained in the activated sludge metabolize the PHB and take up phosphates as biomass increases. More phosphate is taken up by the Bio-P organisms than was previously released, a difference known as luxury uptake.
In addition to the organic and inorganic contaminants described above, waste water typically contains suspended solids in the forms of microorganisms and endogenous mass in the activated sludge and inert organic and inorganic mass in the waste water itself, which must be removed before the liquid is returned to the environment. These solids are typically 0.5 microns or greater in size. In conventional waste water treatment systems the solids are removed from the liquid through gravitational sedimentation and decanting. A quiescent environment is provided for a sufficient amount of time for the solids to separate from the liquid, and then a mechanical decanter withdraws the purified water.
Waste water treatment processes and apparatus have been developed to provide the above-described conditions necessary for the removal of organic contaminants, nitrogen, phosphates and suspended solids. For example, so-called “linear” waste water treatment processes have been developed which comprise a series of tanks or basins in which waste water is sequentially subjected to anaerobic and aerobic conditions, and is then pumped to a clarifier where suspended solids are separated from the purified liquid and the liquid is decanted.
Although this system combines anaerobic, aerobic and clarifying processes, thereby allowing for the removal of organic contaminants, inorganic nutrients and suspended solids, the system is relatively inefficient as it requires numerous tanks, large volumes of liquid and long retention times.
Conventional sequencing batch reactor (SBR) processes have been developed to address these inefficiencies. In an SBR process, waste water is mixed with activated sludge, exposed to intermittent aerobic and anoxic/anaerobic conditions and further purified through settling and decanting in a single vessel. This is typically accomplished through a series of cycles including fill, react, settle and decant phases.
In the fill phase, waste water enters the reactor where it is mixed with activated sludge therein. Aeration is typically intermittent to promote aerobic or anoxic/anaerobic conditions. In the react phase, influent flow is terminated while mixing and aeration continue. Again, aeration is typically intermittent to allow for the removal of nitrogen and phosphorous. In the settle phase, mixing and aeration cease and solids/liquid separation takes place under quiescent conditions. In the decant phase of the cycle, the mixer and aeration systems remain off and the decantable liquid volume is removed by means of a subsurface withdrawal. A typical SBR process cycle requires four to eight hours to complete.
Two reactors may be used in parallel to permit a continuous flow to the system. The total time required for the mixed fill and react fill phases in the first reactor is equal to the time required for the react, settle and decant phases in the second reactor. In this process, 100% of the flow is directed to each reactor 50% of the time. For example, flow typically enters the system for 24 hours of each day. Each reactor would receive flow for a 12-hour period. In a given 12-hour period, the first reactor may be receiving waste water, in the mixed fill and react fill phases, while the second reactor is not receiving flow, as it is in the react, settle or decant phases. At the end of the 12-hour period, the second reactor has completed its treatment cycle and is ready to receive flow for 12 hours. The first reactor begins the react, settle and decant phases.
While conventional SBR processes as described above offer significant advantages over linear and other known waste water treatment processes, such as more efficient settling, decreased operational costs and higher treatment efficiencies, conventional SBR processes nevertheless have shortcomings. For example, the settle phase of each cycle requires quiescent conditions for an appropriate period of time to allow solids/liquid separation, which can require several hours. And in some applications, particularly where the amount of solids is large or the settling characteristics of the solids are imperfect, the settling process does not adequately separate solids from the liquid. Furthermore, the decant phase of each cycle typically requires the use of an expensive mechanical device to extract the desired effluent quality.
The present invention eliminates the need for a mechanical decanter device and the necessity of settle and decant phases, while still accomplishing the removal of organic matter, nitrogen, phosphorous and suspended solids to achieve effluent quality achieved in conventional SBR systems. Specifically, the invention uses a membrane filtration device in conjunction with an SBR process to accomplish solids/liquid separation while at the same time providing alternating, mixed aerobic and anoxic/anaerobic conditions which optimize effluent quality. Since the settle and decant phases of the cycle are eliminated, greater volumes of waste water can be treated in less time as compared with conventional SBR systems. An entire treatment cycle can be completed in approximately two hours. The membrane also increases the efficiency of solids/liquid separation. Adequate separation is achieved even with large amounts of suspended solids that have imperfect settling characteristics. Moreover, because a mechanical decanting device is not needed, construction, operation and maintenance costs are less than those associated with conventional SBR systems.